(1) Field of the Invention
This invention relates to superplastic alloys, especially those made primarily of aluminum. More particularly, the invention relates to the cladding of superplastic alloys to modify and improve their surface characteristics.
(2) Description of the Related Art
Superplastic alloys are crystalline metals that can be deformed well beyond their usual breaking point of less than 100%, and that may be stretched by at least 200%, and often by more than 1000%, during tensile deformation at elevated temperatures. Sometimes, reference is made to metals having “enhanced plasticity”. Such metals typically have deformation properties at the low end of the range of “superplastic” metals, but are still capable of extending more than similar conventional metals. It should be understood that the present discussion encompasses metals having both superplastic and enhanced plasticity. For the sake of convenience, only the term “superplastic” will be used in the following to refer to metals of both kinds.
Superplastic metals elongate and become thinner in a very uniform manner when drawn under tension rather than forming a “neck” (i.e. a local narrowing) which leads to fracture. Instead of forming a neck, the material eventually fails by the slow coalescence of small internal voids until a continuous crack is developed. Such metals usually have a fine grain crystalline structure (e.g. less than 10 micrometers) with a fine dispersion of thermally stable particles which act to pin the grain boundaries and maintain a fine grain structure at high temperatures. The fine grain size is necessary to allow the characteristic deformation mode known as “grain boundary sliding” to occur. The aluminum alloys which show this property generally have a high content of alloying elements, e.g. magnesium, copper or zinc. Typical examples are alloys such as AA5083, AA7075 and Supral® alloys produced by Superform USA of Riverside, Calif. (typically Al, 6 wt. % Cu, 0.4 wt. % Zr). The Mg-containing alloys of the AA5000 series are the most popular ones for producing automotive components. Superplastic alloys of the AA7000 series (containing less Mg—e.g. as low as 1.9 wt %—but high contents of Zn) are currently more popular for aerospace applications.
Superplastic alloys may be used to form objects of complex shape by the application of pressure by means of a gas or with a forming tool, and often with the help of dies (e.g. by means of the Quick Plastic Forming (QPF) process). Aluminum and titanium parts are often superplastically formed for aerospace and, increasingly, automobile applications.
The state of superplasticity is achieved at high temperature, typically more than half the absolute melting point of the alloy concerned, and often around 500° C. (and generally above 400° C.) in aluminum-based alloys. Unfortunately, the commercially relevant Mg-containing alloys (in particular) become susceptible to oxidation and/or surface deterioration during such processing and also during service due to their high content of Mg. These alloys may also become difficult to join together to construct into parts for automobiles and the like after the high temperature forming operation.
Consequently, there is a need to modify or improve superplastic alloys to avoid such problems.
U.S. Pat. No. 4,411,962 which issued to Robert M. Johnson on Oct. 25, 1983 discloses the formation of a metal laminate including one or more layers of superplastic material metallurgically bonded to one or more layers of non-superplastic material in order to achieve high strength while retaining superplastic properties. Bonding of the layers is carried out by diffusion bonding (heated to temperatures below the melting points of the metal) or roll bonding (sheets are rolled together to reduce their thickness and to promote bonding).
U.S. Pat. No. 3,206,808 which issued to Grover C. Robinson on Sep. 21, 1965 relates to the continuous or semi-continuous casting of ingots of aluminum and aluminum alloys. It does not, however, involve the treatment of superplastic alloys.